1. Field of the Invention
The present invention relates to a regulated infrared source, and more particularly, to an infrared source which has a temperature detector and feedback mechanism for maintaining the output of the infrared source at a substantially constant energy density.
2. Description of the Prior Art
Infrared energy is commonly used in detection devices for measuring desired parameters. For example, the infrared energy emitted by a target can be measured to determine the absolute temperature of that target. Thus, the infrared energy emitted by a patient can be measured by a device such as an infrared tympanic thermometer and used to determine the patient's body temperature, as described by some of the present inventors in U.S. patent application Ser. No. 07/570,205. In addition, the differential absorption effects of certain constituents at infrared wavelengths make it possible to determine the constituents of materials such as respiratory gases using an infrared capnograph detection system of the type described, for example, by some of the present inventors in U.S. patent application Ser. Nos. 07/522,177 and 07/522,208. As described therein, infrared light at wavelengths readily absorbed by a particular constituent of a patient's expired air may be passed through the patient's expired air and the degree of absorption measured to determine whether and to what extent that particular constituent is present in the expired air.
Infrared capnograph detection systems of the type described in the aforementioned patent applications operate most efficiently when the infrared source is stable and efficient yet small, lightweight and safe. Although infrared sources (otherwise known as black body radiation sources) have been generally available in the art for quite some time, suitable infrared sources have been unavailable for detecting devices which require a high degree of source stability when there are wide variations in ambient temperature, as during clinical use. Typical prior art black body radiation sources are described, for example, by M. C. Banca et al. in U.S. Pat. No. 3,138,697 and by A. G. De Bell et al. in U.S. Pat. No. 3,205,343. As described in these patents, infrared sources have been designed which operate at temperatures up to 2000.degree. C. or even 2500.degree. C.; however, source stability was not critical to such systems.
Other, more stable, infrared sources have been designed for use in spectrometers and the like for use in infrared spectral analysis. Such infrared sources are described, for example, in U.S. Pat. Nos. 4,499,382 to Vincent; 4,620,104 to Nordal et al.; 4,644,141 to Hagen et al. and 4,935,633 to Curbelo et al. However, these infrared sources also are not sufficiently stable for use in infrared detection devices of the type described in the aforementioned patent applications in that they frequently must be recalibrated.
The energy outputs of prior art infrared sources have been kept relatively constant in a variety of different ways. For example, Curbelo et al. teach that if the source is configured to have a small surface area radiating element which is energized by a high frequency AC square wave, where the frequency of the AC square wave is preferably much greater than the inverse of the heater filament's thermal time constant, then the infrared source's output may be kept relatively constant. However, infrared sources of the prior art remain susceptible to ambient temperature variations which cause temperature drift and subsequent drifts in the emission spectra of the infrared sources as shown in FIG. 1 for the case of a black body radiator used in an infrared gas analyzer. Unfortunately, such temperature drift is unacceptable in infrared capnograph detecting systems of the type described in the aforementioned patent applications. Accordingly, an infrared source which can maintain its output constant for a long period of time without recalibration is highly desirable.
Other techniques are known in the art for stabilizing infrared sources against ambient temperature variations. For example, the infrared source may be placed in an environment maintained at a substantially constant ambient temperature. On the other hand, as described by McClatchie et al. in U.S. Pat. No. 4,103,174, the infrared source can be stabilized against temperature variations by using a radiation loading scheme in combination with a selected source environmental temperature sensitivity without using a temperature controller to maintain the ambient conditions constant. In accordance with the radiation loading scheme of McClatchie et al., the temperature of the infrared source may be maintained relatively constant by providing an infrared radiator which has as high an emissivity as possible and a structure in view of the source which also has a high emissivity so as to prevent reflection back to the source. Since the radiation loading scheme of McClatchie et al. tends to provide a background temperature insensitive to the infrared source, the device of McClatchie et al. may maintain the temperature of the source relatively constant without special heaters or temperature controllers. However, as would be apparent to those skilled in the art, McClatchie et al. maintain the temperature of the infrared source relatively constant by controlling the mounting of the source rather than designing a source which is itself relatively insensitive to ambient conditions.
Other techniques have been taught for providing infrared sources with relatively constant outputs for varying ambient conditions. For example, Brown teaches in U.S. Pat. No. 3,394,259 that a regulator may be used in conjunction with the infrared reference source to control the current to the infrared source and hence the temperature and amount of emitted radiation by disposing a thermistor within the winding of the infrared source. In particular, the thermistor is located within the heater wire and the measured temperature is fed back to a regulator which, in turn, regulates the flow of current to the heater in accordance with variations in the detected temperature, thereby maintaining the emitted infrared energy at a relatively constant temperature level. Brown teaches that best results are achieved when the wire wound body has an emissivity as close to unity as possible. However, although relatively stable, the infrared source of Brown is relatively large and fragile and is thus unsuitable for use as the infrared source in infrared capnographs of the type described in the aforementioned patent applications. Moreover, the temperature detection by the device of Brown is not very accurate since the thermistor can only detect the local temperature at a particular point on a coil which is suspended in air and thus has widely varying temperatures throughout its length.
Accordingly, the present inventors have set out to provide an infrared source which is highly stable, very efficient and very small so that it can fit within a sensor package that is small, lightweight and safe. Preferably, the infrared source temperature may be maintained under servo control and is efficient, easy to manufacture, small in size and rugged. The present invention has been designed to meet these needs.